docs: reproducible doxygen docs using nix (VowpalWabbit#4425)

* docs: reproducible doxygen docs using nix

* remove fixup, add libs, dont include verbatim headers

.github/workflows/build_docs.yml

@@ -24,23 +24,15 @@ jobs:
   cpp:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
-        with:
-          submodules: recursive
-      - name: Configure
-        run: |
-          sudo apt update
-          sudo apt install doxygen graphviz
+      - uses: actions/checkout@v3
+      - uses: cachix/install-nix-action@v18
       - name: Build docs
-        run: |
-          cd doc
-          doxygen --version
-          doxygen
+        run: nix build --print-build-logs .#vw-cpp-docs
       - name: Upload built docs
         uses: actions/upload-artifact@v1
         with:
           name: cxx_docs
-          path: doc/html/
+          path: result/html/
   dump-options-build:
     container:
       image: vowpalwabbit/ubuntu1804-build:latest

CMakeLists.txt

@@ -120,7 +120,6 @@ option(VW_INSTALL "Add install targets." OFF)
 option(BUILD_ONLY_STANDALONE_BENCHMARKS "Build only the benchmarks that can run standalone (and do not use vw internals)" OFF)
 option(BUILD_JAVA "Add Java targets." Off)
 option(BUILD_PYTHON "Add Python targets." Off)
-option(BUILD_DOCS "Add documentation targets." Off)
 option(BUILD_EXPERIMENTAL_BINDING "Add targets for experimental bindings" OFF)
 option(LTO "Enable Link Time optimization (Requires Release build, only works with clang and linux/mac for now)." Off)
 option(RAPIDJSON_SYS_DEP "Override using the submodule for RapidJSON dependency. Instead will use find_package" OFF)
@@ -232,10 +231,6 @@ if (BUILD_FLATBUFFERS)
   add_subdirectory(utl/flatbuffer)
 endif()
 
-if(BUILD_DOCS)
-  add_subdirectory(doc)
-endif()
-
 if(BUILD_JAVA)
   add_subdirectory(java)
 endif()